Hi,
Our house is powered by 2 x 90 Ah 12.6 V Victron Lithium Smart battery packs connected in series to form a nominally 25.6 V battery system. The installation includes a Victron Ve.bus BMS which is connected to a multi-plus and 2 battery protects (1 for DC charging and 1 for DC loads). Recently I have become aware that the performance of at least 1, possibly 2 battery cells, is very different from the other 6.
I became aware of this when the BMS detected 2 separate events; an under-voltage on one of the cells which resulted in the BMS shutting the system down, and an overvoltage which resulted in the charging becoming temporarily de-activated. At first, this appeared to be due to an imbalance in SOC of the cells.
To rebalance the cells I took the system off-line and charged each of the 12.6 V battery packs individually using a laboratory power supply with a charge rate 3A and an absorption voltage of 14.2 V, which was maintained for 2 hours. After the rebalancing process, the battery packs were reconnected and the system brought back on-line.
Over the following days, I monitored the system closely and ensured that a 2 hr absorption period occurred each day. Not only did the batteries quickly become unbalanced again, but I also noticed that the internal resistance of 2 of the cells was quite different from the others.
To systematically test this I captured the cell voltages for the following 3 conditions: Absorption (28.4 V, ~0.3 A), Discharge (13 A) and Bulk (20 A). These three conditions were tested in this order spaced apart by about 1 minute. The results are shown in the attached photos.
To me, this behaviour indicates that the problematic cells are damaged/bad out of the factory, however, I would like some advice from Victron staff with a more in-depth understanding of this product.
These cells have only been in service for about 7 months. They have been cycled at a rate far less than once per day and have not been subject to any charge or discharge currents greater than 45 A (C/2). Current greater than 20 A has not even occurred frequently.
Thanks in advance
Will
